Breathing and Exchange of Gases - Notes | Class 11 | Part 1: Human Respiratory System


Respiration is the oxidation of nutrients in the living cells to release energy for biological work.Breathing is the exchange of O2 from the atmosphere with CO2 produced by the cells.

  • General body surface: E.g. lower invertebrates (sponges, coelenterates, flatworms etc).
  • Skin or moist cuticle (cutaneous respiration): E.g. earthworms, leech, amphibians etc.
  • Tracheal tubes: E.g. insects, centipede, millipede, spider.
  • Gills (Branchial respiration): E.g. fishes, tadpoles, prawn.
  • Lungs (Pulmonary respiration): E.g. most vertebrates.

It consists of a pair of air passages (air tract) and lungs.

1. Air passages:

Conducting part which transports the atmospheric air into the alveoli, clears it from foreign particles, humidifies and brings the air to body temperature.
External nostrils → nasal passage → nasal chamber (cavity) → pharynx → glottis → larynx → trachea → primary bronchi → secondary bronchi → tertiary bronchi → bronchioles → terminal bronchioles → respiratory bronchiole → alveolar duct.
Each terminal bronchiole gives rise to many very thin and vascularised alveoli (in lungs).

A cartilaginous Larynx (sound box or voice box) helps in sound production.

During swallowing, epiglottis (a thin elastic cartilaginous flap) closes glottis to prevent entry of food into larynx.

Trachea, all bronchi and initial bronchioles are supported by incomplete cartilaginous half rings.

2. Lungs:

Lungs situate in thoracic chamber and rest on diaphragm.

Right lung has 3 lobes and left lung has 2 lobes.

Lungs are covered by double-layered pleura (outer parietal pleura and inner visceral pleura).

The pleural fluid present in between these 2 layers lubricates the surface of the lungs and prevents friction between the membranes.

Lungs= Bronchi + bronchioles + alveoli.

Alveoli and their ducts form the respiratory or exchange part of the respiratory system.

Alveoli are the structural and functional units of lungs.

Steps of respiration 
  1. Pulmonary ventilation (breathing).
  2. Gas exchange between lung alveoli & blood.
  3. Gas transport (O2 transport & CO2 transport).
  4. Gas exchange between blood & tissues.
  5. Cellular or tissue respiration.
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